1. Field of the Invention
This invention relates in general to sewage treatment processes and various apparatus for performing those processes. More particularly, this invention relates to an improved liquid aerating rotor assembly of the type used in an aerobic bacterial activated sludge process of sewage treatment.
2. Description of the Prior Art
With the advent of relatively strict governmental controls regarding the pollution of the environment, including the pollution of the nation's water supply, sewage treatment processes and apparatus have become increasingly important. At one time it was customary for sewage to merely be dumped into any available water supply as a means of disposal. This practice by in large is no longer tolerated. It is often now required that the sewage be purified before being deposited into a river or other water source to a state where the water returned to the natural water source is more pure than the water originally extracted therefrom.
One well known type of sewage treatment process which has a number of variations is that which is known generally as aerobic processes. A specific type of aerobic process which is widely used is the aerobic bacterial activated sludge process. In these processes, it is important to aerate or mix the sewage to increase the oxygen content thereof to facilitate the sewage treatment process.
Aeration and mixing devices of various types are well known in the art. A particularly useful device is illustrated in U.S. Pat. No. 3,759,495 which is assigned to the assignee of this invention. This patent discloses a liquid aerating rotor assembly which employs a support frame on which a rotor is rotationally mounted. The rotor rotates about a horizontal axis and has a plurality of mixing surfaces which lift, aerate, and mix the liquid sewage. The rotor assembly is preferably mounted by a plurality of floats to float on top of the liquid sewage. In addition, the rotor assembly is often braced in a fixed position in a race-track or channel type of sewage pond or basin. The rotor induces a flow in the sewage basin which ensures complete mixing of the sewage in conjunction with the aeration imparted to the sewage by this same rotor.
While the rotor assembly shown in the above-noted patent is effective for performing the aerating and mixing function for which it is designed, a number of problems are associated with the use thereof. For one thing, in northern climates where the temperatures during the winter often goes below freezing for extended periods of time, it is not unusual for the surface of the sewage pond or the like to contain relatively large chunks of ice. Such chunks or fragments of ice are present even in those sewage ponds which tend to completely freeze whenever the ice in the pond is broken up either mechanically or during the annular spring thawing. In addition, it is also not unusual for sewage ponds to contain relatively large chunks of debris therein. Thus, when the rotor assembly shown in the above-noted patent is moved or pulled across the surface of the pond, the ice or other debris present in the surface layer of liquid tends to jam the rotor. In extreme cases, this debris will break the rotor blades requiring that the rotor element be replaced. Since the rotor element is often extremely expensive, this is a disadvantageous situation. In addition, the need to replace the rotor element means that a certain amount of down time will be encountered during which the sewage treatment plant or process is inoperative.
One other drawback is sometimes encountered by the device noted in the above patent. While this device is effective in mixing and aerating the sewage, it often has to work harder to accomplish this function because it tends to lift and aerate only the surface layer of liquid. Since the surface layer is the most oxygen rich layer in the pond, an undue amount of horsepower or work must be expended in order to further increase the oxygen content thereof. While various deflecting members may be used to direct the aerated sewage coming from the rotor off to one side or the other so as not to re-aerate that same layer immediately, these deflecting members are still not effective in drawing or causing sub-surface layers of liquid to be directly drawn into the rotor.